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Date: Fri, 10 Sep 2010 10:15:46 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
X-X-Sender: torvalds@i5.linux-foundation.org
To: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
cc: LKML <linux-kernel@vger.kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Hugh Dickins <hughd@google.com>, Christoph Lameter <cl@linux.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        Andi Kleen <andi@firstfloor.org>, linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH 3/4] hugetlb, rmap: fix confusing page locking in
 hugetlb_cow()
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References: <1284092586-1179-1-git-send-email-n-horiguchi@ah.jp.nec.com> <1284092586-1179-4-git-send-email-n-horiguchi@ah.jp.nec.com>
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On Fri, 10 Sep 2010, Naoya Horiguchi wrote:
>  
> -	if (!pagecache_page) {
> -		page = pte_page(entry);
> +	/*
> +	 * hugetlb_cow() requires page locks of pte_page(entry) and
> +	 * pagecache_page, so here we need take the former one
> +	 * when page != pagecache_page or !pagecache_page.
> +	 */
> +	page = pte_page(entry);
> +	if (page != pagecache_page)
>  		lock_page(page);

Why isn't this a potential deadlock? You have two pages, and lock them 
both. Is there some ordering guarantee that says that 'pagecache_page' and 
'page' will always be in a certain relationship so that you cannot get 
A->B and B->A lock ordering?

Please document that ordering rule if so.

			Linus